There is a long recognised need in the art to replace synthetic materials and, in particular, synthetic binders, with more environmentally friendly, natural alternatives. Starch has often been proposed as a “green” alternative to polymers such as latex. However, starch is very different from a latex polymer. When solubilised in water, starch forms a colloidal solution. By contrast, synthetic latex polymers are used in the form of water emulsions. As such, no-one has yet been able to find an entirely satisfactory way of replacing synthetic latex polymers with starch.
U.S. Pat. No. 3,779,857, for example, describes the manufacture of carpet under-layers from a synthetic latex containing large amounts of an inorganic pigment and to which a ternary mixture of granular starch, urea and borax is added in an amount up to 40 parts by weight, based on 100 parts by weight of dry polymer present within the latex. The synthetic latex further comprises a solvent plasticizer such as petroleum oil. During the drying and curing process of the layer formed from the latex, the granular starch gelatinizes. The patent observes the need to prevent an excessively high viscosity of the starch when mixed with the aqueous synthetic latex and the filler. As such, the starch is not pre-gelatinized and is preferably modified by oxidation or acid hydrolysis.
U.S. Pat. No. 4,104,213 does suggest the use of a pre-gelatinized starch. It specifies however that the starch should not be completely gelatinized or fragmented. This partial gelatinization is achieved through the use of steam that is added periodically during a hold time sufficient to give desired volume in a swollen volume test. The starch is then milled. So, the gelatinization should be just sufficient to prevent loss of granular character, as U.S. Pat. No. 4,104,212 of the same patentee and filed on the same day admits. This patent mentions that the latex compositions comprising the starch have a viscosity that is stable during 3 days. However, a stability of 3 days would not be sufficient for a commercial product, and the limited, partial gelatinization suggests a preparation process with limited robustness. This is inherently problematic for industrialisation.
U.S. Pat. No. 4,138,518 proposes to gelatinize the starch in situ in the presence of the synthetic latex polymers in the course of drying the formulation of latex polymers and starch. It is immediately added that a starch sensitizer, including borax, is to be used to lower the gelatinization temperature. The patent further mentions that improvement of the strength and durability of a composite textile coated with latex is significantly improved, when latex containing the starch particles is heated and dried to a temperature and under conditions to cause the starch particles to swell substantially uniformly throughout the thickness of the composite. These statements of the need for lowering the gelatinization temperature and the need for choosing conditions to obtain substantially uniform swelling implies that it is not simple to obtain a product with the desired properties.
EP0670348 proposes the use of un-gelatinized starch as a filler to replace common calcium carbonate fillers in carpet backing compositions. This application admits that it is of essential importance that the latex composition has the proper water content so that the granular structure of the starch product is completely or largely retained. A swelling inhibitor such as urea is therefore very important. If the granular structure were not retained, there would be no filling effect and the carpet will have a larger vulnerability to liquids and to bacteria. Unfortunately, urea results in a reduction of overall adhesiveness of the latex composition.
There is therefore a clear need in the art for a starch-based composition which can be used to replace latex without some of the difficulties identified above and which could mirror some, if not all, of the benefits of a latex emulsion, namely being available as a ready-to-use liquid product, having a relatively low viscosity even at high dry substance, having good visco-stability and providing high wet strength when used, for instance, as a binder. The present invention provides such a composition.